1. Field of the Invention
The present invention concerns an optical vibration sensor and in particular a sensor in which light reflected by a mirror attached to a member subject to the vibrations the parameters of which are to be measured is analyzed.
A vibration sensor of this kind is used to measure acceleration in the bars constituting the stator of a turbo-alternator, for example.
2. Description of the Prior Art
Observation indicates that the bars constituting the stator of a high power turbo-alternator can, over time, become subject to play caused by Laplace electromagnetic forces in association with progressive loosening of the retaining systems.
These forces are expressed mathematically by the product of the length L of the bar by the vector product of the current I flowing through the bar by the magnetic field B to which the bar is subjected.
These forces generate vibrations at twice the frequency of the current. If the turbo-alternator produces a 50 Hz current, the vibration frequency is therefore 100 Hz.
Nevertheless, other vibration frequencies may appear during maneuvers or changes to the operating conditions of the machine.
It is useful to be able to measure the frequency and the amplitude of vibration to which the bars of a turbo-alternator is subjected on a continuous basis, in order to be able to analyze phenomena that occur, to make a record of them and to schedule maintenance operations.
In the light of the foregoing, it would be beneficial to be able to use a "wideband" vibration sensor to obtain accurate information on the vibration status of various locations examined and to carry out accurate amplitude, phase and frequency measurements.
The presence of a high voltage on various bars of the stator makes it impossible to dispose on them conventional sensors connected galvanically to their electronic circuit.
Optical fibers are insulative and it is only natural to use them to produce a vibration sensor for use in areas in which a high electric voltage is present.
One aim of the present invention is therefore to provide a fiber optic vibration sensor for measuring acceleration on a given axis with great accuracy and in a difficult environment.
One example of the design specifications required of the vibration sensor would be:
Measured frequency range: 10 Hz-1 500 Hz. PA1 Measurement phase-shift at 100 Hz: &lt;10 minutes of angle. PA1 Measurement dynamic range at each frequency: 0.1 g to 100 g. PA1 Measurement accuracy: .+-.2% PA1 Transverse sensitivity: &lt;0.1 PA1 Magnetic fields in the order of 1 Tesla. PA1 High voltages in the order of 25 kV. PA1 Relative pressure of hydrogen in the order of 4 bars. PA1 Operating temperature between +10.degree. C. and 120.degree. C. with an ambient temperature variation of as such as 50.degree. C. in 10 minutes. PA1 Finally, the dimensions and the mass of the sensor must not exceed values incompatible with the intended application; in the case of a sensor used to determine vibration of turbo-alternator bars, the dimensions of the sensor must be approximately 15 mm.times.15 mm.times.30 mm, for example. The mass of the sensor is preferably less than 20 grams.
The environment in which the sensor would have to operate to meet the requirements as stated above would be as follows:
In looking to produce an optical vibration sensor, consideration has been given to the use of a sensor relying on the deflection of a built-in (or cantilever) beam by the inertial force due to its own mass: this deflection is measured optically, by directing light onto a reflecting surface attached to the vibrating beam and measuring the optical intensity collected after reflection at the mirror.
The state of the art in this field is defined by documents U.S. Pat. No. 4,581,528 and U.S. Pat. No. 4,649,271, which describe a vibration sensor in which a reflecting built-in beam subject to the vibrations to be measured receives incident light from an optical fiber and reflects this light into the same optical fiber.
The device described in the above two documents has various disadvantages.
Firstly, the vibrating beam is attached by glueing, which makes it fragile and unable to withstand the high operating temperatures or the temperature gradients referred to above.
To make the measurement independent of background illumination, in particular light back-scattered by the interfaces encountered by the incident light before it is reflected at the reflecting beam, the above documents propose to provide the beam with photo-luminescent means for producing a photo-luminescent signal and to analyze at the same time the light reflected by the beam and the photo-luminescent signal in order to deduce the luminous intensity actually reflected by the beam. This analysis requires the presence of members for deflecting the light, highly accurate optical filters and two measurement electronic systems. The analysis device must also include a lens to focus the photo-luminescent signal, the intensity of which is very low. It follows from the foregoing that the prior art sensors are both fragile and costly.
An aim of the invention is to provide a rugged and economic sensor.